The present invention relates to a novel device and method for separating solids from fluid streams or colloidal suspensions. As used herein, “fluid” includes both liquid and gas. A permeable filter medium comprised of passages and openings is used to separate solids from the fluid portion of the colloidal solutions or suspensions in a process known as filtration. A variety of sizes and types of solids are of interest in these separations, ranging in size from a few nanometers, to hundreds of microns and ranging in nature from soft organic solids such as proteins to hard inorganic solids like silica particles. Nano-filters, ultrafilters, microfilters, and macrofilters are examples of permeable media suitable for filtration of particles of a variety of sizes. These filter media, can be comprised of nonwovens, wovens, or perforated screens or meshes produced by known methods in the art including, but not limited to fiber spinning, stretching, wetlaying, phase inversion, entangling fibers lithography, weaving, particle sintering or coalescence.
Although filtration has been employed to separate solids from fluids for many years, an ever present problem of plugging or fouling of the filter remains. Methods of enhancing fluid flow rates in the presence of solids can be found in the prior art. These include: the shearing of liquid slurry across the filter in tangential flow i.e. crossflow filtration, and/or the generation of shear by vibrating the membrane as described in U.S. Pat. No. 4,952,317. Such methods use a pump to force the feed slurry to flow tangentially to the filter, or a mechanical motion of the membrane to generate shear at the membrane surface. The resulting sheer can sometimes cause the concentrated solids to be removed from the face of the filter increasing the rate of fluid flow through the filter. Unfortunately, the provision of pumps or application of mechanical motion to the membrane can require expensive and bulky equipment and require expensive hard plumbing rated to high pressure or capable of withstanding vibrational or mechanical fatigue.
U.S. Pat. No. 4,253,962 proposes the use of sonic vibration, created by ultrasonic transducers, to produce cavitation at the face of the membrane. U.S. Pat. No. 4,526,688 proposes a shock-type system where the support and filter apparatus are mechanically impacted to remove the solids from the filter. U.S. Pat. No. 4,545,969 oscillates a shear plate parallel to a fixed filter surface. Further, U.S. Pat. No. 3,970,564 teaches a system where hard mounted filters are mechanically vibrated normal to their surface. U.S. Pat. No. 5,985,160 demonstrates a device where a solid plate is vibrated up and down near the surface of a filter to improve fluid flow in the presence of solids.
The techniques of cross-flow microfiltration, ultrafiltration, and nanofiltration are generally limited to low shear rates under ˜20,000 s−1. Achieving higher shear rates requires specialized equipment such as that described for example in U.S. Pat. No. 4,952,317 or 5,985,160 and can be difficult to practically achieve. Furthermore such techniques focus primarily on solids removal via driving motion tangent to the filter surface or by vibrating the surface in place.
Alternately, U.S. Pat. No. 5,928,414 and references therein teach back pulse techniques wherein the flow direction is reversed to break up a filter cake which slowly accumulates overtime. While these techniques can be effective, they require flow reversal or shut down of forward flow both of which have significant drawbacks in lost operating time, inefficient back flow of valuable filtered fluid, and high energy costs.
A filtration device which is able to autogenously dislodge accumulated solids from the face of the filter surface while maintaining a continuous flow through the filter without a need for tangent flow, vibration, or flow reversal would be a useful advance in the art of filtration and separation.
As used herein, “autogenous cleaning” means self-cleaning during use.
The applicants have discovered a filtration method in which solids are dislodged and ejected from the filter surface without interruption to forward flow via switching an elastically supported filter sheet between a slack and ballooned state during filtration. To enable transition from the aforementioned slack state to the ballooned state, the filter medium must must obey the relation Tmedium>Esupport(Aballooned/Aslack). Here Tmedium=Tensile Strength of the sheet, Esupport=Elastic Modulus of the support, Aballooned=the geometric area of medium surface in the ballooned state, and Aslack=the geometric area of the filter medium in the slack state. In addition, the applicants have discovered that increasing the frequency of transition from slack to ballooned states, and increasing the ratio of area ballooned/area slack improves the fluid flow through the filter during filtration.